@Arkandel In games with less steep and less step-y power curves, I think a more gradual approach would make sense. (The Reach's approach, after all, was extremely gradual -- arguably too much so by the end of the game when the end state was closing in on 1,000 XP.) There are probably some level based games that have both of these features -- I just tend to associate the term with the Dungeons & Dragons model, where the gulf between a level 2 character and a level 10 character is so vast that they might as well be playing separate games. It's less an issue of catchup and more an issue of how I can provide the player base with stuff to do; that task is a lot simpler when someone who wants to run an adventure can run up a flag and be assured that everyone who responds will be close enough in level to one another that they can form a workable group.
The Flashback idea was sort of a spur of the moment thing, so the details are still on the hazy side. What I wanted was a way to work new characters into the game a little more organically -- they don't just pop into existence fully formed at level 15 with the expectation that existing characters will trust them on adventures against world-shattering danger. Instead, they'd play some fixed number of adventures at gradually escalating level, so maybe they have one at 3rd, 6th, 9th, and 12th levels, and then they come into play at the "low" tier of the current game level (15-17, in this example). Then I realized it would be a lot easier to get players together if a game had something like the WoW retro-raid feature that automatically scales your character and ilvl back to the point where something like the Auchenai Crypts is an appropriate challenge instead of a joke. Which led me to the idea of just snapshotting every character's sheet whenever they level, so you can easily roll back to the days when you were 5th level to do Flashback adventures with new characters.
What I like about this is that there's something for everyone. People get to establish backstory with each other in a low-stress environment (the older characters must have survived the adventure, after all, if they're still alive in the present), and staff doesn't have to get headaches from the people at 16th level complaining because they're running stories for the newbies again. And, importantly, the characters always exist as peers -- there never needs to be a situation where someone's irrelevant to the story because he's too low or too high level.
It does introduce some problems; even if you don't let existing characters die in flashbacks, there's still the possibility of some truly epic continuity snarls. But if you're doing pulp high adventure, that's probably not high on your list of worries.
